In oil and gas wells, over time, there is usually also a certain production of water. The water production typically increases with time and may after a while become so large that further production of hydrocarbons is no longer remunerative. The water can be naturally present in the reservoir. It can also be water injected from another well to maintain the production and the pressure in the reservoir. The produced water includes chemical compounds some of which may be harmful for the environment and must be removed before the water eventually may be discharged to the sea.
Some of the chemicals are dissolved in the water from nature's side while others are added and dissolved as production chemicals. Production chemicals are thus included in the produced water and constitute an environmental problem.
Produced water is thus not only an economic disadvantage for the oil companies but also a significant environmental challenge. In many cases produced water is reinjected to maintain the pressure. This also involves costs. The best technical and economical solution would be to shut off the water selectively in the reservoir which is the object of the present agent and method.
The prior art technology in this area covers a broad range of suggestions of use of water swellable polymers to block the flow of water through permeable zones.
There are water swellable polymers based on positively or negatively charged (ion active) monomers and polymers based on non ion active monomers. Some polymers are particulate during injection but are stretched out to linear polymers when swelled in water. Some of these can again have functional groups like PO4, which are intended to form some kind of bond (hydrogen bond or ion bond) or adhesion to the surface of the formation.
In general it is positive for the ability of the polymer to take up water that it is not strongly cross-linked but able to be converted to a substantially linear polymer when swelled in water. This way many water molecules may be associated to the polymer chain. On the other hand the same property (low degree of cross-linking) is negative for the stability of the polymer both with respect to remaining in position within the formation and with respect to maintaining the desired properties, such as viscosity and general chemical stability with respect to chemical influences from the environment.
With regard to polymers of negatively charged ions these have a significant disadvantage in the form of insufficient swellability in environments where salts or acids are present. The same may be said, though to a somewhat less extent, about polymers based on positively charged monomers. Neutral systems are influenced least by salt concentrations.
U.S. Pat. No. 5,701,955 (Framton, 1997) teaches a method and a dispersion for use to reduce water permeability in subterranean formations. The dispersion is comprised by water swellable polymer particles being produced by emulsifying polymerization of inverted (reverse) phase (water-in-oil), in which at least 90% of the particles have a size less than 10 μm. Cross-linkers are not given any particular attention in this patent. The water swellable polymer particles of this patent exhibit excellent swelling ability but the particles loose most of their stability during swelling as there are few water-stable bonds present and there is no occurrence of additional cross-linking in the swollen condition to stabilize the particles.
U.S. Pat. No. 6,454,003 B1 and the US patent applications NOs. 2003/0116317 A1, 2003/0149212 A1 and 2003/0155122 A1 (Chang et al.) teach the production of water swellable polymer particles produced in a reverse-phase emulsion polymerization. The particle swelling occurs upon breakage of labile cross-linking bonds initiated by pH change and increased temperature in an aqueous phase. Two types of cross-linkers ensure that swelling does not occur until the particles have reached the desired location in the formation but do not influence or increase the stability of the system as swelled. Furthermore the method described by Chang (water-in-oil) can not be used to capsulate solid, water soluble components with the object of later formation of a stable system, since the water soluble components would be dissolved in the discontinuous phase (water) during the particle formation involving chemical reactions already during the particle production. The desired reactions should only take place subsequent the placement in the subterranean formation and such systems can only be produced in oil-in-oil emulsion systems. A main object of the present invention is the formation of a stable, particle based system enduring high pressures and temperatures and in which the resulting water swelled system is held in place and remains pressure and temperature stable while blocking water (minimizing water permeability) by chemical reactions taking place subsequent to the system having been placed in the formation and has come into contact with formation water.
PCT patent application with publication No. WO A1 02/40828 describes a system which is based on cross-linked hydrophilic particles manufactured in a water-in-oil based emulsion process with subsequent polymerization to solid particles. The particles comprise water soluble scale inhibitors (dissolved in the discontinuous phase, water, during manufacture) and the particles are evenly distributed in a sand screen around the production well. When water is produced, the scale inhibitor is released from the particles in contact with water. The particles have a high degree of cross-linking and portion matrix as the particles are not to be deformed when the inhibitors have been released but maintain their form ad location as part of the sand screen. No gelling or water blocking is described or mentioned as desired according to this publication. The particles constitute an amount of only 10% of the volume of sand screen and the permeability is maintained. WO A1 02/40828 describes chelating (complex forming) chemicals like AMPS, vinyl phosphoric acid derivatives etc. since these molecules have scale inhibiting properties. It is these properties that are used according to this publication.
PCT patent application WO A1 02/14453 describes a method for stabilizing porous sand formations for a limited period of time to allow drilling and subsequent casing with a steel casing in the well without well cave-in (collapse). The method according to this publication makes use of chemicals to glue together sand grains in the formation but the process is not supposed to reduce the permeability of the formation. The chemicals used shall provide adhesion between sand particles present without blocking pores. WO A1 02/14453 does not describe an agent or a method for reducing or eliminating permeability. It is therefore not suitable for water blocking or the like. WO A1 02/14453 furthermore describes a system which over time is supposed to degrade and in which the components from the degradation process or substances added during the “placing process” are supposed to control reservoir phenomena like wax, scale, hydrates and moisture properties.
U.S. Pat. No. 6,169,058 B1 concretely concerns fracturing of subterranean formations using liquids of high viscosity. The viscosity of the liquids is controlled by addition of a per se known viscosity increasing agent. Furthermore the water swellable, hydrophilic polymer particles used are cross-linked and therefore not water soluble. The patent is not directed towards the particles as such. The primary object of the polymer particles is to reduce loss of fluids such as circulating drilling fluids in the formation.
PCT patent application with publication No. WO 98/06929 describes a method and a composition for reducing water permeability in subterranean formations. The composition is based on at least a vinyl amide monomer of a certain formula, a monomer comprising ammonium or quaternary ammonium units as well as a cross-linking monomer. The method involves injecting the particles into the formation in question by means of a carrier fluid and an inverting surfactant, the particle size being assumed to be of a magnitude less than the pore size of the formation. Zwitterionic monomers are not mentioned. It is clearly emphasized in this publication and the examples that the swellable particles are manufactured in an inverted (water-in-oil) polymerization process.
Even though many agents and methods have been described in the literature to shut off water in permeable zones in subterranean formations, there is still a need for new systems which are improved in the sense that they are better suited for all types of environments, not dependent upon depth, temperature, the rock species, acid or salt concentrations and which are able to meet any requirements of delaying the swelling process and in which the particle size during production can be adapted to the pore size or the permeability of the formation in question.